1. Field of the Invention
The present invention relates generally to the field of display devices, and more specifically, to a method and apparatus for centering an active video signal on a display.
2. Background Information
A display device typically receives a video signal and converts the same into an image for a user to view. The video signal may be received from a variety of sources such as a microprocessor in a computer system, a storage media, or a television transmission. One common display device is the cathode ray tube (CRT) display device.
FIG. 1 illustrates a typical CRT. In CRT, an electron gun 104 shoots an electron beam 108 through a set of magnets 112 to a fluorescent screen 116. When electrons within the electron beam 108 strike the fluorescent screen 116, the energy of the electrons excites a phosphorous coating causing the fluorescent screen 116 to emit light photons 120.
The set of magnets 112 direct where the electron beam 108 strikes the fluorescent screen 116. Typically, the set of magnets 112 includes a top magnet 124 and a bottom magnet 128, and side magnets 132 and 136. Top magnet 124 and bottom magnet 128 move electron beam 108 from the top 130 to the bottom 131 of the fluorescent screen 116, while side magnets 132 and 136 move electron beam 108 in a horizontal direction 138 across fluorescent screen 116. Each sweep of the electron beam 108 in a horizontal direction 138 forms a scan line 140 on fluorescent screen 116. Once a scan line 140 has been formed, the magnets are controlled to move the electron beam to a new starting position 141, allowing the side magnets 132 and 136 to again sweep the electron beam across the surface of the fluorescent screen 116 to form a subsequent scan line 142. By rapidly moving the electron beam across the screen, a series of scan lines forms an image for display on the fluorescent screen 116.
Electronics 144 control the electromagnetic fields created by the set of magnets 112. Electronics 144 also control the firing of electron gun 104. The video electronics 144 typically controls the electron gun by processing video signals arriving from a processor such as a microcomputer 148. However, the current electronics 144 typically have difficulty processing the video signal to precisely center an image. Therefore, the video signal is typically centered manually by adjusting physical controls on the display. Such a technique is imprecise and cumbersome. In addition, continuous or sporadic manual adjustments may be required.
A bi-sectional technique is also employed to converge on a value that centers the active video signal. The bi-sectional technique involves an iterative trial and error process where the picture is moved by a small step in one direction (left or right) until the picture is centered in the middle of the screen 116. However, this technique typically takes several seconds to arrive at the target position. In order to comply with the Microsoft PC-99 Design Guide, the time for signal switching must be completed within two seconds. As such, the bi-sectional technique does not provide an adequate solution for converging on the target value by the required time.
Accordingly, there is a need in the technology for overcoming the drawbacks described above.